The avalanche photodiode (APD) is a semiconductor device that can detect extremely low levels of electromagnetic radiation. Unlike a PIN photodiode, which generally produces a single electron for each photon received, an APD is constructed so that an electron dislodged by a photon will hit other atoms in the APD semiconductor lattice with sufficient velocity and energy so that additional hole-electron pairs are created by the collisions. Typically a free electron will create a number of hole-electron pairs, and the electrons from these pairs will, in turn, create additional electrons, thus creating an "avalanche" process. This multiplication of the electrons gives the APD an effective gain and allows the detection of very low light levels.
Recent advances in the fabrication and performance of the avalanche photodiodes (APD's) have led to their use in the detection of individual photons and other short-duration events. Although APD's have obvious advantages over photomultiplier tubes, including power requirements, size, and quantum efficiency, until recently, photomultiplier tubes have been required to get sufficient sensitivity and speed in these applications. When used in the single photon detection applications, APD's are frequently used in "Geiger" mode in which the APD is reverse biased to a voltage that exceeds its breakdown voltage. In geiger mode, some means is necessary to stop or "quench" the current flowing through the diode after each avalanche.
One method to quench the current is to limit the maximum current flowing through the diode, by means of a series resistor, to a low enough level that the current will spontaneously cease due to the statistical nature of the avalanche process. While using this simple circuitry, the minimum interval between detectable events is limited by the so-called "dead time": the time required to turn off the diode completely and to recharge it, and any other parasitic or intrinsic capacitance associated with the diode, through the typically large current limiting resistor which results in a large RC time constant.
These so-called passive quenching circuits are reverse biased through a biasing means such as a series resistance by applying a high voltage, V.sub.RB, comprised of the breakdown voltage, V.sub.BR, plus the overvoltage .DELTA.V across the avalanche device of which avalanche photodiodes are but one example. When an event such as a thermal energy application or impingement of a photon occurs in the case of an avalanche photodiode the avalanche current begins to flow, the junction between the resistance means for biasing and the avalanche photodiode rises toward .DELTA.V, and the voltage across the photodiode approaches the breakdown voltage V.sub.BR. Eventually the voltage at the junction stanches the avalanche current. The system will only reach full sensitivity when the discharge is completed and reset in the time dictated by the RC time constant which is typically long.
To shorten the resetting time active quench circuits were developed which, for example, may use a comparator to sense the onset of an avalanche current and through the action of a monostable circuit apply a voltage of .DELTA.V plus an excess voltage V.sub.X to the junction of the biasing resistor and avalanche photodiode to drive it safely below V.sub.BR and stop the avalanche current. And after a short delay, typically applied through another monostable circuit, a switch is closed to ground from that junction to quickly recharge the intrinsic capacitance of the avalanche photodiode. Although this reduces the "dead time" by circumventing the RC time constant delay suffered by the passive quenching circuits it introduces substantial parasitic or intrinsic capacitance. This additional capacitance increases the charge flow through the avalanche diode and adds to the heating effect too. These circuits also must operate at low signal levels to detect the avalanche onset at the earliest moment which makes them more sensitive to noise. Because they are using a high speed comparator, these circuits require more power to detect and stop the avalanche process. One example of an active quench circuit is shown in U.S. Pat. No. 5,532,474.